The present invention relates to a process for preparing alkali metal or alkaline earth metal tricyanomethanides having a particularly high purity, in particular a high freedom from halides.
Ionic liquids containing alkali metal or alkaline earth metal tricyanomethanides (TCMs) are important raw materials and auxiliaries in the electrical industry, for example for the production of rechargeable batteries and electronic building blocks. For these fields of application, it is necessary that the methanides used are particularly pure, in particular free of halides, in order to avoid corrosion problems and/or unwanted secondary reactions.
The preparation of tricyanomethanides is known. The cyanidation of malononitrile (MN) was first described by Schmidtmann in Ber. Dtsch. Chem. Ges. 1896, 29, 1168. Here, MN is deprotonated by means of sodium ethoxide in ethanol and converted into sodium tricyanomethanide by stepwise addition of chlorocyan and then crystallized from ether. Sodium tricyanomethanide is isolated in a yield of about 70% in the process.
In Chem. Ber. 1929, 62B, 153, Birckenbach et al. have disclosed the cyanidation of MN by means of bromocyan. Both Birckenbach et al. and Mayer et al. (Monatsh. Chem., 1969, 100, 462) have described the preparation of silver tricyanomethanide which is low in halogen by admixing crude alkali metal tricyanomethanide with silver nitrate, resulting firstly in precipitation of silver chloride or silver bromide. After further addition of silver nitrate, silver tricyanomethanide could be isolated from the filtrate. Furthermore, Mayer et al. have described the reaction of silver tricyanomethanide with chlorocyan at 100° C. for 40 hours to give tetracyanomethane which was then sublimed and subsequently hydrolyzed in sulfuric acid to form ammonium tricyanomethanide. Mayer et al. obtained lithium tricyanomethanide by addition of lithium chloride to an acetonitrile solution of tetracyanomethane at −96° C.
The preparation of high-purity potassium tricyanomethanide was disclosed for the first time by Hipps et al. (J. Phys. Chem. 1985, 89, 5459). In this process, potassium tricyanomethanide was recrystallized ten times. To remove residual organic impurities, the potassium tricyanomethanide obtained was subsequently recrystallized twice from water. This gave a white crystalline powder which on excitation with light at 5145 Å displayed no Raman fluorescence background and was interpreted as highly pure.
The cyanidation of MN by means of bromocyan in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO®) in THF has been disclosed in WO-A-98/29389. In this process, DABCO® hydrochloride crystallizes out at −20° C. over a period of 28 hours. 98% pure lithium tricyanomethanide was obtained here.
A further process has been disclosed by Trofimenko et al. in J. Org. Chem. 1962, 27, 433, and in this potassium tricyanomethanide was obtained by treatment of a dihalomalononitrile-potassium bromide complex with potassium cyanide. The process gives yields of up to 60%.
In Bull. Soc. Chim. Fr. 1954, 948, Fox et al. have described a further process for preparing tricyanomethane at low temperature in which bromomalononitrile was reacted with potassium cyanide.
Further processes for preparing tricyanomethanides, which comprise reaction of deprotonated MN with phenyl cyanate, have been disclosed by Grigat et al. in Chem. Ber. 1965, 98, 3777-3784 and Martin et al. in DD-A-48614. Yields of from 75 to 88% were obtained here.
However, the known processes do not give products which are halogen-free and the products always have to be after-treated in more or less complicated purification steps.
One process for purifying sodium tricyanomethanide by recrystallization in acetonitrile was described in 1987 by Bock et al. in Z. Naturforsch., 1987, 42b, 315. Here, sodium tricyanomethanide was obtained in a yield of 70%. However, the purity obtained is not evidenced by precise information.
It was an object of the present invention to provide a simple and quick process for preparing highly pure tricyanomethanides. The process should be suitable for industrial production.